Electroluminescent devices are opto-electronic devices which light emission is produced in response to an electrical current through the device. The physical model for EL is the radiative recombination of electrons and holes. The term light emitting diode (LED) is commonly used to describe an EL device where the current-voltage behaviour is non-linear, meaning that the current through the EL device is dependent on the polarity of the voltage applied to the EL device. Both organic and inorganic materials have been used for the fabrication of LEDs. Inorganic materials such as ZnS/Sn, Ga/Bs, Ga/As have been used in semiconductor lasers, small area displays, LED lamps, etc. However, the drawbacks of inorganic materials include difficulties to process and to obtain large surface areas and efficient blue light.
Organic polymers used as light-emitting materials in EL devices offer several advantages over inorganic materials, such as simpler manufacturing, low operating voltages, the possibility of producing large area and full-color displays. Conjugated polymers such as poly(phenylvinylene) (PPV) were first introduced as EL materials by Burroughes et al in 1990 (Burroughes, J. H. Nature 1990, 347, 539). Considerable progress has been made since then to improve the stability, efficiency, and durability of polymeric LEDs (Sheats, J. R. et al Science 1996, 273, 884; Berggren, M. et al Nature 1994, 372, 444-6; Holmes, A. B. et al WO 94/29883 (1994); and Spreitzer, H. et al, Adv. Mater. 1998, 10(16), 1340). Polymers with wide energy bandgap to emit blue light are important materials because stable, efficient blue-light-emitting materials with high brightness are desirable for full color EL display applications. With these primary materials, it is possible to produce other colors by a downhill energy transfer process. For instance, a green or red EL emission can be obtained by doping a blue EL host material with a small amount of green or red luminescent material.
Incorporating an efficient chromophore into a polymer side chain is an effective approach to obtain blue light emitting polymer. The main chain of the polymer can provide necessary physical properties such as mechanical and film-forming properties and the side chain provides desired electro-optical properties. Blue-light-emitting polymers (Li et al. Syn. Met. 1997, 84, 437) have been prepared by this approach. However, the polymers show high threshold voltages and operating voltages. Thus, it is desirable to develop processable new blue-light-emitting polymers with low driving voltages for full color display.